This invention relates generally to electronic type platform weighing systems, and more particularly to a free-standing scale having an improved base member for aligning parts of the scale.
There are many different types of electronic weighing scales in use today. One popular type of electronic weighing scale is constructed with a platform for receiving the load to be weighed and a set of levers, pivots, flexures, and torque tubes to mechanically combine the forces applied to the platform by the load. The combined forces are then applied to a single electronic load cell to yield the weight of the load. The load cell is typically constructed with a mechanically-deformable sensor plate which operates as a force transducer. The sensor plate includes one or more sensor elements that serve to convert the mechanical bending forces of the sensor plate into electrical signals. When a load is applied to such a load cell, the sensor elements produce electrical signals which are proportional to the load applied to the load cell.
Many load cells utilize a measurement beam which carries all or a part of the load to be measured and thus deforms as a function of the weight of the load. Load cell measurement beams are typically either of two types, bending beams or shear beams. Bending beams undergo bending strains that vary as a function of the weight of the load applied to the beams, while shear beams undergo shear strains that vary as a function of the weight of the load applied to the beams. Strain measuring devices, such as strain gauges or the like, are normally mounted on the beams to measure the magnitude of the load induced bending strains in bending beams or the load induced shear strains in shear beams.
The accuracy of load cells employing bending beams and shear beams is highly dependent on the manner in which the beams are supported and/or how the loads are coupled to the beams. Ideally, changes in the load induced deformation of the beam, i.e., the bending strain or shear strain, should be solely a function of changes in the weight of the load. If the structure that either supports the beam or couples the load to the beam applies rotational moments or twisting torques to the beam, then the deformation of the beam will not be a true indication of the weight of the load.
Not only should the beam be supported and/or loaded in a manner that does not apply rotational moments or twisting torques to the beam, but the beam supporting or loading structure should not restrain the beam from the load induced deformations that are to be measured. For example, for a beam that is freely supported at each end, i.e., a non-cantilever beam, the support structure should allow the ends of the beam to freely pivot.
The location at which the beam is supported and/or the location where the load is applied to the beam can also affect the accuracy of load cells using measurement beams. In particular, it is important that the beams be symmetrically supported and loaded so that the weight induced deformation of the beam is symmetrical.
The foregoing problems in the art can exist in any weighing scale that employs measurement beams, and can be especially exasperated by the placement of the scale on an uneven support surface. As a result of supporting the weighing scale on an even surface there can be large variations in both the direction and the location that the load is applied to the bending beams and shear beams through the support structure.
In the past, attempts have been made to ensure the proper direction and location of beam support and loading by either using complex and costly mechanical coupling mechanisms or by attempting to electrically compensate for the inaccuracies. For example, in U.S. Pat. No., 4,554,987, a scale assembly is provided that includes a platform which is supported by a plurality of force transmitting assemblies. The force transmitting assemblies and platform cooperate to automatically center the platform relative to an enclosing structure and to align the force transmitting assemblies and platform. The automatic centering of the platform and aligning of the force transmitting assemblies is accomplished by moving the platform back and forth in sideways directions against stops which limit motion of the platform. Centering the platform and aligning the force transmitting assemblies is claimed to be effective to eliminate sideward force components on load cells.
In U.S. Pat. No. 6,177,638, a portable load scale is disclosed for use in rugged terrain or at locations without suitable support pads. The load scale includes a support deck affixed to a base platform through a plural number of load cells. The base platform is constructed to provide ramp members joined by longitudinal runner assemblies to form a rigid, non-flexing assembly having a central gap and gaps between pairs of ramp members to reduce the standard rectangular footprint by approximately thirty percent. The runner assemblies are constructed so that the bottom of the support deck is separated from the top of the base plate of the runner assemblies by a distance of several inches. The load cells are mounted onto the underside of the support deck and joined to the base platform by ball bushings such that the load cells can pivot in any or about all axis directions relative to the base platform to relieve stresses induced by uneven terrain.
None of the prior art weighing systems have proved to be wholly satisfactory, especially when the weighing system is also to be portable, light weight, and of a size that is appropriate for table top applications. There remains a need for an improved structure that supports the beams, or couples the load to the beams, to reduce or prevent the application of unwanted rotational moments or twisting torques to the beam system, so that the deformation of the beam will be a true indication of the weight of the load.
In one embodiment of the invention, a foot assembly for a weighing scale is provided including a base having a cylindrical wall and at least one blind hole. An annular ring is arranged in coaxial spaced relation to the base, and supported by a plurality of resilient beams that project radially outwardly from a portion of the base. Each of the resilient beams comprises a compound curve contour so that the plurality of resilient beams may twist and/or bend so as to take up and compensate for the resultant unwanted rotational moments or twisting torques that result from placement of the foot assembly on an uneven surface.
In another embodiment of the invention, a weighing scale is provided including a platform coupled to a mounting tray, where the mounting tray has a plurality of apertures. A weight determination assembly is positioned between the platform and the mounting tray. A plurality of force transfer beams are arranged within the mounting tray so as to substantially support the platform and the mounting tray such that the mounting tray is isolated from a support surface. In this way, forces that are applied to the weighing scale by the placement of a load on the platform are transferred to the plurality of force transfer beams, without direct interaction between the mounting tray and the support surface. A plurality of foot assemblies are positioned within the apertures and operatively interconnected to the plurality of force transfer beams. Each of the foot assemblies includes a base having a plurality of compensation beams that project radially outwardly so as to support an annular ring that is coupled to the mounting tray. In this way, if a support surface onto which the weighing scale is placed is canted at some angle, the compensation beams twist and/or bend so as to take up and compensate for any unwanted rotational moments or twisting torques.